DWR's Final Construction Report Spec 65-09 documents these specific details of the DWR field engineer overriding the construction contractor and telling the contractor to only excavate "to grade" level per the drawings. This means that any section of the spillway that had clay seams would "remain" (See FR post link below).
The contractor was following the original DWR designer's spec's to dig to "sound rock", and if the "sound rock" was below "grade" then the contractor was to place "backfill concrete" to restore the slope to the original base "grade" that the slabs would be built upon.
Why did the DWR field engineer make this critical "change"?
Again, the DWR Final Construction Report Spec 65-09 reveals that someone, at DWR, did anchor bar testing that included clay, as this was "the worst foundation available". Since this test yielded a chosen 30,000 tensile psi pull strength (likely when the clay was hard and dry), the construction orders were changed away from excavating to fresh bedrock.
The interim Independent Forensic Team (IFT) report noted this general issue, but worded it in a subtle & gentle way such that you had to discern the severity of how bad this issue destabilized the spillway from high pressurized cracked slab water flows under the spillway - and its impact to the poor clay foundation areas - to its ultimate destruction.
See related & more details at link: (Note: the IFT's interim report recently set the record straight regarding DWR's insistence that the original spillway was built upon "good competent rock". Ouch): DWR tells another Whopper? Reporter corners DWR on FERC PFMA on Spillway - DWR says full spillway was built on "good competent rock"
OK, thanks.
Is this also the same area where we now have to deal with the natural spring?